Selenium dioxide (SeO2) Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, polar or nonpolar
SeO2 is the chemical formula for representing selenium dioxide, a colorless, lustrous crystalline solid. It is used as an oxidizing agent and a colorant in the chemical manufacturing, glass-making, and printing industry.
The IUPAC name for SeO2 is selenium (IV) oxide, as Se has a +4 oxidation state in this compound. If you are curious to know the chemistry behind SeO2, then this article is for you.
In this article, you will learn how to draw the Lewis dot structure of SeO2, what is its molecular geometry or shape, electron geometry, bond angles, hybridization, formal charges, and/or whether SeO2 is a polar or a non-polar molecule.
The Lewis structure of SeO2 consists of a selenium (Se) atom at the center. It is double-covalently bonded to two oxygen (O) atoms at the sides. There are a total of 3 electron density regions around the central Se-atom in this Lewis structure comprising 2 Se=O bond pairs and 1 lone pair of electrons on the central atom.
If you want to learn how to draw the SeO2 Lewis dot structure in the best and simplest way, then follow the steps given below.
There are two different elemental atoms present in SeO2, i.e., a selenium (Se) atom and two oxygen (O) atoms. When you look through the Periodic Table of elements, you will find that both selenium and oxygen are located in Group VI A (or 16). This denotes that both Se and O consist of a total of 6 valence electrons in each atom.
∴ The SeO2 molecule consists of 1 Se-atom and 2 O-atoms. Hence, the total valence electrons in the Lewis dot structure of SeO2 = 1(6) + 2(6) = 18 valence electrons.
2. Choose the central atom
In this second step, usually the least electronegative atom out of all the concerned atoms is chosen as the central atom.
This is because the least electronegative atom is the one that is most likely to share its electrons with the atoms spread around it.
As selenium (Se) is less electronegative than oxygen (O) so, the Se-atom is placed at the center of the SeO2 Lewis structure while the two O-atoms are spread around it, as shown below.
3. Connect outer atoms with the central atom
At this step of drawing the Lewis structure of a molecule, we need to connect the outer atoms with the central atom using single straight lines.
As the two O-atoms are the outer atoms in the Lewis structure of selenium dioxide (SeO2), both the oxygen atoms are joined to the central Se-atom using single straight lines, as shown below.
Each straight line represents a single covalent bond containing 2 electrons.
Now, if we count the total valence electrons used till this step out of the 18 available initially, there are a total of 2 single bonds in the structure drawn above. Thus, 2(2) = 4 valence electrons are used till step 3.
Total valence electrons available – electrons used till step 3 = 18 – 4 = 14 valence electrons.
This means we still have 14 valence electrons to be accommodated in the Lewis dot structure of SeO2.
4. Complete the octet of the outer atoms
There are two O-atoms present as outer atoms in the Lewis structure of SeO2. Each O-atom needs a total of 8 valence electrons in order to achieve a stable octet electronic configuration.
A Se-O single bond already represents 2 electrons, which means both the O-atoms require 6 more electrons each to complete their octets. Thus, these 6 valence electrons are placed as 3 lone pairs around each O-atom, as shown below.
5. Complete the octet of the central atom and convert a lone pair into a covalent bond if necessary
Total valence electrons used till step 4 = 2 single bonds + 2 (electrons placed around O-atom, shown as dots) = 2(2) +2(6) = 16 valence electrons.
Total valence electrons available – electrons used till step 4 = 18-16 = 2 valence electrons.
So the remaining 2 valence electrons are placed as a lone pair on the central Se-atom.
However, a problem here is that the central Se-atom still has an incomplete octet. It consists of 2 single bonds and 1 lone pair that denotes a total of 6 valence electrons, consequently a deficiency of 2 more electrons in order to achieve a stable octet electronic configuration.
So, to solve this issue, a lone pair present on any one of the two outer O-atoms is converted into an additional covalent chemical bond between the central Se-atom and the respective O-atom, as shown below.
Now, you may notice that the central Se-atom has a complete octet with 1 single bond + 1 double bond + 1 lone pair = 8 valence electrons. The octet of both terminal O-atoms is also complete.
But is this Lewis structure stable? Let’s check that using the formal charge concept in the next step.
6. Check the stability of the SeO2 Lewis structure using the formal charge concept
The fewer formal charges present on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charges can be calculated using the formula given below.
The above calculation shows that as per the SeO2 Lewis structure obtained so far, the central Se-atom carries a +1 formal charge while the single-bonded O-atom carries a -1 formal charge.
But wait a minute; we learned that the fewer the formal charges, the more stable the Lewis representation of a molecule. So can we reduce Se and O formal charges?
The good news is that we can do so by converting another lone pair into a covalent chemical bond.
7. Minimize formal charges by converting another lone pair into a covalent chemical bond and again check the stability of Lewis’s structure
At this step, a lone pair on the single-bonded O-atom is converted into an extra bond between the central Se-atom and the respective O-atom, as shown below.
In this way, the central Se-atom is uniformly bonded to two O-atoms via double covalent bonds.
According to the above Lewis structure, the central Se-atom now has a total of 10 valence electrons surrounding it.
This situation falls under the expanded octet rule. Atoms such as sulfur, phosphorus and selenium can accommodate more than 8 valence electrons owing to the presence of a d-subshell in their outermost shell.
After completely filling the 4p-subshell in Se, the extra incoming electrons start occupying 4d orbitals.
Both the terminal O-atoms have a complete octet with 1 double bond +2 lone pairs each in the above structure.
Finally, we again need to check the stability of the Lewis structure obtained in step 7 by applying the formal charge formula to ensure that overall formal charges are minimized, as expected.
Zero formal charges on all three bonded atoms denote that there is no overall charge present on the SeO2 Lewis structure; thus, it is the most stable and correct Lewis representation of the selenium dioxide molecule.
What are the electron and molecular geometry of SeO2?
The ideal electron pair geometry of SeO2 is trigonal planar. However, it is due to the presence of a lone pair of electrons on the central Se-atom that SeO2 adopts a different shape or molecular geometry, i.e., bent, angular, or V-shaped.
Molecular geometry of SeO2
The selenium dioxide (SeO2) molecule has a bent shape or molecular geometry. The presence of a lone pair of electrons on the central Se-atom leads to strong lone pair-bond pair repulsions in addition to a Se=O bond pair-bond pair repulsive effect in the molecule.
The strong electronic repulsions distort the shape and geometry of the molecule. The Se=O bond pairs tilt inwards, away from the lone pair at the top, to minimize the electron repulsive effect. It thus results in a bent, angular, or V-shape, as shown below.
Always keep in mind that the molecular geometry or shape of a molecule gets strongly influenced by the different number of lone pairs and bond pairs around the central atom.
Contrarily, the ideal electron pair geometry only depends on the total electron domains around the central atom (lone pairs and bond pairs inclusive).
A double covalent bond such as Se=O is considered a single electron domain while determining the electronic geometry of SeO2.
Electron geometry of SeO2
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule containing 3 regions of electron density around the central atom is trigonal planar.
In SeO2, there are two Se=O bonds and one lone pair around the central Se-atom that makes a total of 3 electron density regions or electron domains. Thus, the ideal electron pair geometry of SeO2 is trigonal planar.
A shortcut to finding the electron and the molecular geometry of a molecule is by using the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it.
It is used to predict the shape and geometry of a molecule based on the VSEPR concept.
AXN notation for SeO2 molecule
A in the AXN formula represents the central atom. In SeO2, a selenium (Se) atom is present at the center, so A = Se for SeO2.
X denotes the atoms bonded to the central atom. In SeO2, there are two Se=O bonds; thus, X=2.
N stands for the lone pairs present on the central atom. As per the Lewis structure of SeO2, there is one lone pair of electrons on the central selenium atom, so N = 1.
Hence, the AXN generic formula for the selenium dioxide molecule is AX2N1.
Now, you may have a look at the VSEPR chart given below.
The VSEPR chart confirms that the molecular geometry or shape of an AX2N1-type molecule is bent or V-shaped while its ideal electron pair geometry is trigonal planar, as we already noted down for the selenium dioxide (SeO2) molecule.
Hybridization of SeO2
The central Se-atom and both O-atoms are sp2 hybridized in SeO2.
The electronic configuration of a selenium atom is 1s22s22p63s23p63d104s24p4.
During SeO2 chemical bonding, one 4p electron of selenium shifts to its empty 4d atomic orbital. Two 4p atomic orbitals of selenium then hybridize with its 4s atomic orbital to produce three sp2 hybrid orbitals.
The sp2 hybrid orbital containing paired electrons is situated as a lone pair on the central Se-atom in SeO2. The other two sp2 hybrid orbitals containing a single electron each form Se-O sigma (σ) bonds by overlapping with the sp2 hybrid orbitals of terminal O-atoms.
The unhybridized p and d atomic orbitals of selenium then form the required pi bonds by overlapping with the p-orbitals of oxygen atoms in each Se=O covalent bond.
Refer to the figure drawn below.
A shortcut to finding the hybridization present in a molecule is by using its steric number against the table given below.
The steric number of central Se-atom in SeO2 is 3, so it has sp2 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The SeO2 bond angle
It is due to the distortion present in the SeO2 molecular shape and geometry that the O=Se=O bond angle decreases slightly from the ideal.
The ideal bond angle in a symmetrical trigonal planar molecule is 120° while the O=Se=O bond angle is approx. 119° in the selenium dioxide molecule. Each Se=O bond length is about 162 pm in SeO2.
The polarity of a molecule is largely governed by the electronegativity difference between bonded atoms and molecular shape or geometry.
As per Pauling’s electronegativity scale, a truly polar covalent bond must have an electronegativity difference of 0.5 to 1.6 units between the bonded atoms.
There is a high electronegativity difference of 0.89 units between a selenium (E.N = 2.55) and an oxygen (E.N = 3.44) atom in each Se=O bond. Thus, both Se=O bonds are strongly polar in the SeO2 molecule. Each Se=O bond thus possesses a specific dipole moment value (symbol µ).
Terminal O-atoms strongly attract the Se=O shared electron cloud. Hence the central Se-atom gains a partial positive (δ+) charge while both the terminal O-atoms gain partial negative (δ–) charges, respectively.
The asymmetrical bent shape of SeO2 further endorses the polarity effect.
The Se=O dipole moments do not get canceled equally in SeO2. The charged electron cloud stays non-uniformly spread over the molecule. Thus, it is overall polar (net dipole moment, µ= 2.62 D).
The Lewis structure of SeO2 displays a total of 18 valence electrons, i.e., 18/2 = 9 electron pairs, including 4 bond pairs and 5 lone pairs.
A selenium (Se) atom is present at the center.
It is double-covalently bonded to two oxygen (O), one on each side of the molecule.
The central Se-atom carries one lone pair of electrons while 2 lone pairs are present on each terminal O-atom.
How many total bond pairs and lone pairs are present in the SeO2 Lewis structure?
There are a total of 4 bond pairs and 5 lone pairs.
Out of the 5 lone pairs, there are 2 lone pairs on each double-bonded O-atom, while 1 lone pair of electrons is present on the central Se-atom in the SeO2 Lewis dot structure.
What is the molecular shape of SeO2?
The selenium dioxide (SeO2) molecule has a bent shape or molecular geometry. It is also known as angular or V-shape.
Why is the molecular shape of SeO2 different from its ideal electron pair geometry?
The presence of a lone pair of electrons on the central Se-atom distorts the shape and geometry of SeO2.
Lone pair-bond pair repulsions exist in the molecule that pushes the bonded atoms away from the lone pair at the center.
The Se=O bonds tilt inwards to form a bent or V-shape, different from its ideal electronic geometry, i.e., trigonal planar.
What is the difference between the shapes of these three dioxides: CO2, SO2, and SeO2?
In CO2, the central C-atom belongs to group IV A (or 14) of the Periodic Table. All four valence electrons of carbon consumed in C=O covalent bonding leave behind no lone pair on the central C-atom in CO2. Thus there is no distortion witnessed in its shape and/or geometry.
In contrast, both SO2 and SeO2 consist of a central atom belonging to group VI A (or 16) of the Periodic Table. Four electrons consumed in bonding out of the 6 initially available results in a lone pair of electrons on the central atom.
The presence of a lone pair of electrons makes the molecule adopt a bent shape, different from its ideal trigonal planar geometry.
What is the bond angle between oxygens in the SeO2?
The O=Se=O bond angle is 119° in the SeO2 molecule. It is slightly reduced from the ideal bond angle of 120° due to the distortion present in the shape and geometry of selenium dioxide.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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